THE SPECIAL SENSES. THE CHEMICAL SENSES: TASTE AND SMELL The receptors for taste (gustation) and smell (olfaction) are chemoreceptors that respond to.

THE SPECIAL SENSES

THE CHEMICAL SENSES: TASTE AND SMELL The receptors for taste (gustation) and smell (olfaction) are chemoreceptors that respond to chemicals in aqueous solution Taste Buds and the Sense of Taste –Taste buds, the sensory receptor organs for taste, are located in the oral cavity with the majority located on the tongue (few scattered on soft palate, inner surface of cheek, pharynx, and epiglottis of larynx) Most found in papillae (peglike projections of the tongue mucosa that give the tongue surface a slightly abrasive feel) –Taste sensations can be grouped into one of five basic qualities: sweet, sour, bitter, salty, and umami (meaty or savory taste produced by monosodium glutamate) –Physiology of Taste: For a chemical to be tasted it must be dissolved in salvia, move into the taste pore, and contact the gustatory hairs Each taste sensation appears to have its own special mechanism for transduction

THE CHEMICAL SENSES: TASTE AND SMELL Taste Buds and the Sense of Taste –Afferent fibers carrying taste information from the tongue are found primarily in the facial nerve and glossopharyngeal cranial nerves –Taste impulses from the few taste buds found on the epiglottis and the lower pharynx are covered via the vagus nerve –Taste is strongly influenced by smell and stimulation of thermoreceptors, mechanoreceptors, and nociceptors

TONGUE

TASTE Sweet: –Elicited by many organic substances including sugars, saccharin, alcohols, some amino acids, and some lead salts (found in lead paints) Sour: –Produced by acids, specifically their hydrogen ions (H + ) in solution Salt: –Produced by metal ions (inorganic) Table salt (sodium chloride) tastes the saltiest Bitter: –Elicited by: alkaloids: organic alkaline substances that react with acids to form salts that are used for medical purposes –Quinine, nicotine, caffeine, morphine, and strychnine Some nonalkaloids: –Aspirin Umami: –Discovered by the Japanese? –Elicited by the amino acid glutamate –Responsible for the beef taste of steak, tang of aging cheese, and the flavor of the food additive monosodium glutamate (sodium salt of glutamic acid)

TASTE Most taste buds respond to two or more taste qualities, and many substances produce a mixture of the basic taste sensations

TASTE BUDS Taste buds are located mainly on the: –Tops of fungiform papillae which are scattered over the entire tongue surface –In epithelium of the side walls of the large round circumvallate (vallate) papillae Largest and least numerous Inverted V shape at back of tongue

TASTE BUDS Gourd (squash) shape Epithelial cells of three types: –Supporting cells: Form bulk of taste bud Insulate the receptor cells from each other and form the surrounding tongue epithelium –Receptor cells: Called taste cells or gustatory cells –Basal cells: Act as stem cells, dividing and differentiating into supporting cells, which in turn give rise to new gustatory cells

TASTE BUDS Gustatory hairs: –Long microvilli that project from the tips of both supporting and gustatory (taste) cells –Extend through a taste pore to the surface of the epithelium –Bathed by saliva –Receptor membranes of gustatory cells

TASTE BUDS Coiling around gustatory cells are sensory dendrites that represent the initial part of the gustatory pathway to the brain –Each afferent fiber receives signals from several receptor cells within the taste bud Because of their location, taste buds cells are subjected to huge amounts of friction and are routinely burned by hot foods –Replace every 7 to 10 days

TASTE MAPS Sweet: tip of tongue Salt: front sides Sour: mid/rear sides Bitter: back Umami: pharynx

TASTE MAPS Maps are dubious (uncertain)

TASTE HOMEOSTATIC VALUES Likes: –Umami guides intact of protein –Sugar and salt helps satisfy the body’s need for carbohydrates, minerals, and some amino acids –Sour rich source of vitamin C (oranges, lemons, tomatoes) Dislike: –Bitter is protective since many poisons and spoiled foods tend to be bitter

GUSTATORY CORTEX

PHYSIOLOGY OF TASTE For a chemical to be tasted it must be dissolved in salvia, diffuse into the taste pore, and contact the gustatory hairs Different gustatory hairs have different thresholds for activation: –Bitter receptors detect substances in very minute amounts –Others are less sensitive

PHYSIOLOGY OF TASTE Transduction: process by which stimulus energy is converted into a nerve impulse: –Each taste quality appears to have its own mechanism Salt is due to Na + influx through sodium channels followed by Ca 2+ influx Sour is mediated by H + which appears to act on a taste cell in one of three ways –Directing entering the cell –Opening channels that allow other cations (+) to enter –Blockage of K + channels Bitter, sweet, and umami are mediated by G protein-dependent mechanisms that act via second messengers to promote depolarization by increasing intracellular levels of Ca 2+ (bitter) or closing K + channels (sweet)

PHYSIOLOGY OF TASTE Gustatory Pathway: cranial nerves –Tongue: Facial nerve (VII): chorda tympani transmits impulses from taste receptors in the anterior 2/3 of the tongue Glossopharyngeal nerve (IX): services the posterior 1/3 and the paharynx just behind –Epiglottis and lower pharynx: Vagus nerve (X) These afferent fibers synapse in the solitary nucleus of the medulla to the thalamus and ultimately to the gustatory cortex in the parietal lobes –Fibers also project to the hypothalamus and limbic system which determine our appreciation of what we are tasting –Initiate reflex synapses with the parasympathetic system increasing salivary secretion in the mouth and gastric secretion in the stomach

Influence of Other Sensations on Taste Taste is 80% smell –When olfactory receptors in the nasal cavity are blocked (congestion/pinching) food is bland Mouth also contains thermoreceptors, mechanoreceptors, and nociceptors (pain: chili peppers) –The temperature and texture of foods can enhance or detract from their taste

THE CHEMICAL SENSES: TASTE AND SMELL The Olfactory Epithelium and the Sense of Smell –The olfactory epithelium is located in the roof of the nasal cavity and contains the olfactory receptor cells –To smell a particular odor it must be volatile and it must be dissolved in the fluid coating the olfactory epithelium –Axons of the olfactory receptor cells synapse in the olfactory bulbs sending impulses down the olfactory tracts to the thalamus, the hypothalamus, amygdala, and other members of the limbic system

OLFACTORY EPITHELIUM Organ of smell is a yellow- tinged patch of pseudostrtified epithelium (olfactory epithelium) located in the roof of the nasal cavity Air entering the nasal cavity must make a hairpin turn to stimulate olfactory receptors before entering the respiratory passageway –Poor location –This is why sniffing, which draws more air superiorly across the olfactory epithelium, intensifies the sense of smell

OLFACTORY EPITHELIUM Olfactory epithelium covers the superior nasal concha on each side of the nasal septum, and contains millions of bowling pin- shaped olfactory receptor cells (surrounded and cushioned by columnar supporting cells, which make up the bulk of the penny-thin epithelial membrane) –Supporting cells contain a yellow- brown pigment similar to lipofuscin (insoluble fatty pigment found in aging cells that they have ingested but not completely digested) which gives the olfactory epithelium its yellow hue At the base of the epithelium lie the short basal cells

OLFACTORY RECEPTORS

Olfactory receptor cells: –Unusual bipolar neurons –Each has a thin apical (apex) dendrite that terminates in a knob from which several long cilia radiate (olfactory cilia) Increase the receptive surface area Covered by a thin coat of mucus produced by supporting cells and by olfactory glands in the underlying connective tissue –Mucus is a solvent that captures and dissolves airborne odorants –Olfactory cilia are largely nonmotile

OLFACTORY RECEPTORS Unmyelinated axons of the olfactory receptor cells gather into small fascicles that collectively form the filaments of the olfactory nerve (cranial nerve I) Olfactory neurons are unique in that they undergo noticeable turnover throughout adult life –Remember that taste receptors are epithelial cells –Their location puts them at risk of damage –Typical life span is 60 days –Replaced by differentiation of the basal cells in the olfactory epithelium

Specificity of Olfactory Receptors Taste is classified into 4 to 5 categories Humans can distinguish approximately 10,000 odors but research suggests that our olfactory receptors are stimulated by different combinations of a more limited number –There are at least 1000 smell genes that are only active in the nose –Each gene encodes an odorant binding protein that responds to several different odors and each odor binds to several different receptor types However, each receptor cell has only one type of receptor protein Pain receptors (irritants, temperatures) send impulses to the CNS by means of the trigeminal nerves

Physiology of Smell Activation Particular odorant must be volatile (gaseous state) Must dissolve in the fluid coating the olfactory epithelium Dissolved odorants stimulate olfactory receptors by binding to protein receptors in the olfactory cilium membranes and opening specific Na + channels –Leads to an action potential (impulse) that is conducted to the olfactory bulb (distal end of the olfactory tracts)

Physiology of Smell Mechanism Transduction (biochemical conversion: docking of specific chemicals to receptors resulting in production of specific enzymes or nerve impulses) Odorant chemical binding to the G protein-associated receptor sets the cyclic AMP (cAMP) second messenger system into motion causing Na + and Ca 2+ channels to open (note Ca 2+ channels not shown) Influx of Na + causes depolarization and impulse transmission

OLFACTORY TRANSDUCTION PROCESS

Olfactory Pathway Axons of the olfactory receptors that constitute the olfactory nerves that synapse in the overlying olfactory bulbs (distal ends of the olfactory tracts)

Olfactory Pathway Filaments of the olfactory nerves synapse with second-order neurons (mitral cells) in complex structures called glomeruli –Axons from neurons bearing the same kind of receptor converge on a given type of glomerulus –Each glomerulus receives only one type of odor signal –Mitral cells: Refine, amplify, relay signal Olfactory bulbs also contain granule cells that inhibit mitral cells contributing to olfactory adaptation –Subconsciously turn off smell (smoke, cabbage, etc.)

Olfactory Pathway Impulses flow from the olfactory bulbs via olfactory tracts (composed mainly of mitral cell axons) to two destinations –1.via thalamus to the piriform lobe of the olfactory cortex and part of the frontal lobe just above the orbit (smells are consciously interpreted and identified) Each olfactory cortical neuron receives input from up to 50 receptors to analyze –2.via the subcortical to the hypothalamus, amygdala, and other regions of the limbic system Emotional response to odors (trigger sympathetic system) –Danger: gas –Attractiveness: perfume Appetizing odors: cause increase salivation and stimulate digestive tract Unpleasant odors: trigger protective reflexes such as sneezing and coughing/choking

THE CHEMICAL SENSES: TASTE AND SMELL Homeostatic Imbalances of the Chemical Senses –Anosmias (loss of smell) are olfactory disorders resulting from head injuries that tear the olfactory nerves, nasal cavity inflammation, or aging 1/3 of chemical sense loss is due to zinc deficiency –Zinc is a growth factor for the receptors of the chemical senses –Uncinate seizures (brain disorders): Some temporal lobe epileptics (repetitive abnormal electrical discharges within the brain) Olfactory hallucinations Psychological Irritation of olfactory pathways (surgery/trauma)

THE EYE AND VISION Vision is our dominant sense with 70% of our body’s sensory receptors found in the eye Nearly half of the cerebral cortex is involved in some aspect of visual processing

THE EYE AND VISION Accessory Structures of the Eye: –Eyebrows are short, coarse hairs overlying the supraorbital margins of the eye that shade the eyes and keep perspiration out –Eyelids (palpebrae), eyelashes, and their associated glands help to protect the eye from physical danger as well as from drying out –Conjunctiva is a transparent mucous membrane that lines the eyelids and the whites of the eyes It produces a lubricating mucus that prevents the eye from drying out –The lacrimal apparatus consists of the lacrimal gland, which secretes a dilute saline solution that cleanses and protects the eye as it moistens it, and ducts that drain excess fluid into the nasolacrimal duct –The movement of each eyeball is controlled by six extrinsic eye muscles that are innervated by the abducens and trochlear nerves

EYEBROWS Short, coarse hairs that overlie the supraorbital margins of the skull Shade the eyes from sunlight Prevent perspiration trickling down the forehead from reaching the eyes Orbicularis oculi muscle depresses the eyebrow when contracted Corrugator muscle contraction moves eyebrow medially

EYELIDS Called palpebrae Separated by palpebral fissure (eyelid slit) –Meet at the medial and lateral commissures (canthi) Medial canthus is the location of the lacrimal caruncle: –Contains sebaceous and sweat glands –Produces the whitish, oily secretion (sandman’s eye-sand)

EYELIDS Thin, skin-covered folds supported internally by connective tissue sheets called tarsal plates –Anchor muscles Upper lid more mobile Eyelid muscles activated reflexively to cause blinking every 3- 7 seconds: –Prevents drying –Spreads accessory structure secretions (oil, mucus, and saline solution) across eyeball surface

EYELIDS Follicles of the eyelash hairs are richly innervated by nerve endings (hair follicle receptors) –Anything that touches the eyelashes (even puff of air) triggers reflex blinking Tarsal glands (Meibomian) –Embedded in the tarsal plates –Ducts open at the eyelid edge just posterior to the eyelashes –Modified sebaceous glands –Produce oily secretion that lubricates the eyelid and eye Prevents eyelids from sticking together –Infection results in cyst (chalazion) Closed sac or pouch containing fluid and solid material resulting from clogged duct Associated with the eyelash follicles are typical sebaceous glands Between the hair follicles are modified sweat glands called ciliary glands Inflammation of sebaceous or ciliary glands results in a sty –Purulent fluid (pus)

CONJUNCTIVA Transparent membrane Produces lubricating mucus that prevents the eyes from drying Lines the eyelids as the palpebral conjunctiva and reflects (folds back) over the anterior surface of the eyeball as the ocular (bulbar) conjunctiva Ocular (bulbar) conjunctiva: –Covers only the white part of the eye not the cornea (clear window over iris and pupil) –Very thin –Blood vessels are clearly visible beneath it More visible in irritated “bloodshot” eyes Conjunctival sac: –Space between conjunctiva-covered eyeball and eyelids –Where contact lens lies Inflammation called: conjunctivitis –Red, irritated eyes –Pinkeye: conjunctival infection caused by bacteria or viruses Highly contagious

Lacrimal Apparatus Consists of the lacrimal gland and the ducts that drain excess lacrimal secretions into the nasal cavity Lacrimal gland lies in the orbit above the lateral end of the eye and is visible through the conjunctiva when the lid is everted (turn inside out) Releases a dilute saline solution called lacrimal secretion (tears) into the superior part of the conjunctival sac through several small excretory ducts –Blinking spreads the tears downward and across the eyeball to the medial commissure –Tears enter the paired lacrimal canaliculi (canals) via two tiny openings called lacrimal puncta (visible as tiny red dots on the medial margin of each eyelid –From the canals the tears drain into the lacrimal sac and then into the nasal cavity at the inferior nasal meatus –Can fill nasal cavity causing congestion (sniffles)

Lacrimal Apparatus Lacrimal fluid contains mucus, antibodies, and lysozyme (enzyme that destroys bacteria) –Cleanses and protects the eye surface as it moistens and lubricates Enhanced tearing serves to wash away or dilute irritating substances Importance of emotionally induced tears is poorly understood Nasal inflammation can constrict the nasolacrimal duct preventing tears from draining from the eye surface causing “watery” eyes

EYE MUSCLES

Movement of eyeball is controlled by six straplike extrinsic muscles which originate from the bony orbit and insert into the outer surface of the eyeball Help maintain the shape of the eyeball and hold it in the orbit

EYE MUSCLES

HOMEOSTATIC IMBALANCE OF THE EYE MUSCLES Diplopia: movements of the external muscles of the two eyes are not perfectly coordinated –Two images instead of one (double vision) –Paralysis, muscle weakness, alcohol Strabismus: cross eyed –Might be due to congenital (present at birth) weakness of the external muscles –Condition in which the eyes rotate either medially or laterally –To compensate the eyes may alternate in focusing or only the controllable eye is used (brain disregards inputs from the deviant eye, which then becomes functionally blind)

THE EYE AND VISION Structure of the Eyeball –Three tunics form the wall of the eyeball The fibrous tunic is the outermost coat of the eye and is made of a dense avascular connective tissue with two regions: the sclera and the cornea The vascular tunic (uvea) is the middle layer and has three regions: the choroid, the ciliary body, and the iris The sensory tunic (retina) is the innermost layer made up of two layers: the outer pigmented layer absorbs light; the inner neural layer contains millions of photoreceptors (rods and cones) that transduce light energy –Internal Chambers and Fluids Posterior segment (cavity) is filled with a clear gel called vitreous humor that transmits light, supports the posterior surface of the lens, holds the retina firmly against the pigmented layer, and contributes to intraocular pressure Anterior segment (cavity) is filled with aqueous humor that supplies nutrients and oxygen to the lens and cornea while carrying away wastes –The lens is an avascular, biconcave, transparent, flexible structure that can change shape to allow precise focusing of light on the retina

INTERNAL EYE STRUCTURES

FIBROUS TUNIC The fibrous tunic is the outermost coat of the eye and is made of a dense avascular connective tissue with two regions: the sclera and the cornea Sclera: –Forming the posterior portion and the bulk of the fibrous tunic (glistening white and opaque) –Protects and shapes eyeball –Provides sturdy anchoring site for extrinsic muscles –Posteriorly, where the sclera is pierced by the optic nerve, it is continuous with the dura mater of the brain

FIBROUS TUNIC Cornea: anterior portion –Crystal clear –Light enters the eye –Light bending apparatus –Covered by epithelial sheets on both faces –Outer epithelial cells (merge with the ocular conjunctiva at the sclera-cornea junction) continuously renew the cornea –Deep epithelial cells have active Na pumps that maintain the clarity by keeping the water content low –Well supplied with nerve pain receptors (problem with contacts) –Capacity for regeneration and repair is extraordinary –Only tissue in the body that can be transplanted with little or no possibility of rejection (no blood vessel-beyond the reach of the immune system)

VASCULAR TUNIC (UVEA) The vascular tunic (uvea) is the middle layer and has three regions: the choroid, the ciliary body, and the iris Choroid: –Highly vascular (blood vessels supply nutrients to the eye) –Dark brown (melanocytes) membrane that forms the posterior 4/5 of the uvea Helps absorb light, preventing it from scattering and reflecting within

VASCULAR TUNIC (UVEA) Ciliary body: –Anterior thicken ring of tissue that encircles the lens –Consist mainly of smooth muscle bundles called ciliary muscles which control lens shape –Posterior surface of the ciliary body forms the ciliary process which contains the capillaries that secrete the fluid that fills the cavity of the anterior segment of the eyeball –Suspensory ligament (zonule) Extends from the ciliary process to the lens Holds the lens in its upright position

VASCULAR TUNIC (UVEA) Iris: –Visible colored part of the eye –Most anterior portion of the uvea –Lies between the cornea and lens –Continuous with the ciliary body posteriorly –Its round opening, the pupil, allows light to enter –Different colors but only brown pigment: Large amount of brown pigment: brown Varying amounts: shorter wavelengths of light are scattered from the unpigmented parts, and the eyes appear blue, green, or gray

PUPIL SIZE IRIS: –Made of two smooth muscle layers with bundles of sticky elastic fibers that congeal into a random pattern before birth These muscle fibers allow it to vary pupil size Pupil dilation is controlled by sympathetic fibers Pupil constriction is controlled by parasympathetic fibers

SENSORY TUNIC (RETINA) The sensory tunic (retina) is the innermost layer made up of two layers: –Outer pigmented layer absorbs light and prevents it from scattering in the eye Also acts a phagocytes and stores vitamin A needed by the photoreceptor cells –Inner neural layer contains millions of photoreceptors (rods and cones) that transduce light energy

RETINA

Neural layer: three types of neurons –1.photoreceptors: rods and cones –2.bipolar cells –3.ganglion cells

RETINA Photoreceptors: –Rods: Dim light and peripheral vision receptors Do not provide either sharp images or color vision Retinal periphery: only rods –Cones: Operate in bright light Provide high-acuity color vision Fovea centralis region (center of the macula lutea): all cones Toward the retina periphery cone density decreases Anything we wish to view critically is focused on the fovea centralis (center of the macula lutea)

MICROSCOPIC ANATOMY OF THE RETINA Light (yellow arrow) passes through the retina to excite the photoreceptors Flow of electrical signals occurs in the opposite direction Local currents are produced in response to light and spread from the photoreceptors to the bipolar neurons and then to the innermost ganglion cells, where action potential is generated Ganglion cell axons leave the posterior aspect of the eye as the thick optic nerve Horizontal and amacrine cells play a role in visual processing Optic nerve exits the eye at the optic disc (blind spot)

POSTERIOR WALL (FUNDUS) OF RETINA

Part of the posterior wall (fundus) of the eye as seen with an ophthalmoscope Note the optic disc from which the retinal blood vessels radiate Note: Macula which is the area of most acute vision

HOMEOSTATIC IMBALANCE OF RETINA Retinal Detachment: –Pigmented and nervous layers separate and allow the jellylike vitreous humor to seep between them Can cause permanent blindness because it derives the neural retina of its nutrient source –Head trauma or jerking of the head (auto/bungee jumping) –Wet/Dry variations

Biconvex, transparent, flexible structure that can change shape to allow precise focusing of light on the retina Enclosed in a thin, elastic capsule and held in place just posterior to the iris by the suspensory ligament Avascular (blood vessels interfere with transparency)

LENS Two regions: –Lens epithelium: Confined to the anterior lens surface –Lens fiber: Formed from the lens epithelium Bulk of lens No nuclei No organelles Contain transparent protein called crystallins –Converts sugar into energy for use by the lens Since new fibers are continually added, the lens enlarges throughout life, becoming denser, more convex, and less elastic. All of which gradually impair its ability to focus light properly

CATARACT

Clouding of the lens Most result from age-related hardening and thickening of the lens or as a secondary consequences of diabetes mellitus Heavy smoking and frequent exposure to intense sunlight Whatever the promoting factors, the direct cause seems to be inadequate delivery of nutrients to the deeper lens fibers resulting in clumping of the crystallin proteins

INTERNAL CHAMBERS AND FLUIDS Internal Chambers and Fluids: –Posterior segment (cavity) is filled with a clear gel called vitreous humor that transmits light, supports the posterior surface of the lens, holds the retina firmly against the pigmented layer, and contributes to intraocular pressure –Anterior segment (cavity) is filled with aqueous humor that supplies nutrients and oxygen to the lens and cornea while carrying away wastes

AQUEOUS HUMOR Anterior segment is partially subdivided by the iris into the anterior chamber (between the cornea and the iris) and the posterior chamber (between the iris and the lens) The entire anterior segment is filled with aqueous humor, a clear fluid similar in composition to blood plasma –Forms and drains continually and is in constant motion –Filters from the capillaries of the ciliary processes into the posterior chamber and freely diffuses –Drains into the venous blood via the scleral venous sinus (canal of Schlemm) –Normally produced and drained at the same rate, maintaining a constant intraocular pressure of about 16 mm Hg, which helps to support the eyeball internally –Supplies nutrients and oxygen to the lens and cornea and to some cells of the retina –Carries away metabolic wastes

VITREOUS HUMOR Posterior segment Clear gel that binds a tremendous amount of water Transmits light Supports the posterior surface of the lens and holds the neural retina firmly against the pigmented layer Contributes to intraocular pressure, helping to counteract the pulling force of the extrinsic eye muscles Forms in the embryo and lasts a lifetime

HOMEOSTATIC IMBALANCE OF HUMORS Glaucoma –Drainage of aqueous humor blocked –Fluid backs up –Pressure in the eye builds up compressing the retina and optic nerve –Result is blindness unless the condition is detected early –Damage can be done before you realize it –Late signs include seeing halos around lights and blurred vision

WAVELENGTH AND COLOR Electromagnetic radiation includes all energy waves, from long radio (meters) to very short gamma waves (nanometers) 1 nm = 10 -9 m Our eyes respond to the part of the spectrum called visible light (wavelength range of 400- 700 nm) Light is packets of energy (photons) traveling in a wavelike fashion at very high speed (186,00 miles per second; 300,000 km/s

VISIBLE SPECTRUM Red wavelengths are the longest and have the lowest energy Violet wavelengths are the shortest and most energetic Objects have color because they absorb some wavelengths and reflect others White reflects all wavelengths Black absorbs all wavelengths Red apple reflects red Green grass reflects green

ELECTROMAGNETIC SPECTRUM

REFLECTION Light travels in straight lines –Easily blocked by any nontransparent object –Like sound, light can reflect, or bounce, off a surface –Reflection of light by objects in our environment accounts for most of the light reaching our eyes

REFRACTION Speed of light traveling in a given medium is constant –Passing from one transparent medium into another with a different density changes its speed Speeds up as it passes into a less dense medium Slows down as it passes into a denser medium –Because of these changes in speed, bending or refraction of a light ray occurs when it meets the surface of a different medium at an oblique angle rather than at a right angle (perpendicular) The greater the incident angle, the greater the amount of bending

REFRACTION OF LIGHT

REFRACTION Image demonstrates the consequence of light refraction when a spoon is placed in a half-full glass of water Spoon appears to break at the air-water interface

LENS Transparent object curved on one or both surfaces Light hitting the curve at an angle is refracted

CONVEX LENS Lens that is thickest in the center (convex) Light rays are bent so that they converge (come together) or intersect at a single point called the focal point The thicker (more convex) the lens, the more the light is bent and the shorter the focal distance (distance between the lens and focal point) Image formed by a convex lens is called a real image –Upside down and reversed from left to right

CONCAVE LENS Thicker at the edges than at the center Magnifying glasses Diverge the light (bend it outward) so that the light rays move away from each other Prevents light from focusing and extends the focal distance

Focusing of Light on the Retina As light passes from air into the eye, it moves sequentially through the cornea, aqueous humor, lens, and vitreous humor, and then passes through the entire thickness of the neural layer of the retina to excite the photoreceptors that abut (border) the pigmented layer Light is bent three times: as it enters the cornea and on entering and leaving the lens –The refractory power of the humors and cornea is constant –The lens is highly elastic, and its curvature and light-bending power can be actively changed to allow fine focusing of the image

Focusing for Distant Vision Aim both eyes so that they are both fixated on the same spot Our eyes are best adapted for distant vision The far point of vision is that distance beyond which no change in lens shape is required for the normal (emmetropic) eye –About 6m or 20 ft Ciliary muscles are completely relaxed, and the lens (stretched flat by tension in the suspensory ligaments) is as thin as it gets

Focusing for Distant Vision (a) Light from distant objects (over 6m away) approaches as parallel rays and, in the normal eye, need not be adjusted for proper focusing on the retina. Image is a real image (inverted and reversed from left to right)

Focusing for Close Vision Light from objects less than 6m away diverges as it approaches the eye and it comes to a focal point farther from the lens Focusing for close vision demands that the eye make three adjustments: accommodation of the lens, constriction of the pupils, and convergence of the eyeballs

Focusing for Close Vision (b) Light from close objects (less than 6m away) tends to diverge and lens convexity must be increased (accommodation) for proper focusing. Image is a real image (inverted and reversed from left to right)

FOCUSING

Focusing for Close Vision Lights from objects less than 6m away diverges as it approaches the eyes and it comes to a focal point farther from the lens Close vision demands that the eye make adjustments To restore focus, three processes– accommodation of the lenses, constriction of the pupils, and convergence of the eyeballs– must occur simultaneously

Focusing for Close Vision Accommodation of the lenses Is a process that increases the refractory power of the lens Near point is 10 cm (4 inches) from the eye The gradual loss of accommodation with age reflects the lens’s decreasing elasticity

Focusing for Close Vision Constriction of the Pupils Circular (constrictor) muscles of the iris reduce the size of the pupil Accommodation papillary reflex, mediated by parasympathetic fibers of the oculomotor nerves, prevents the most divergent light rays from entering the eye –Such rays would pass through the extreme edge of the lens and would not be focused properly (blurred vision)

Focusing for Close Vision Convergence of the Eyeballs The visual goal is always to keep the object being viewed focused on the retinal fovea When we fixate on a close object our eyes converge Controlled by somatic motor fibers of the oculomotor nerves Closer the object, the greater the degree of convergence required –When you focus on the tip of your nose, you go cross- eyed

Focusing for Close Vision Reading or other close work requires almost continuous accommodation, papillary constriction, and convergence Prolonged periods of reading tire the eye muscles and can result in eyestrain –Helpful to look up and stare into the distance occasionally to relax the intrinsic muscles

Homeostatic Imbalances of Refraction Visual problems related to refraction can result from a hyperrefractive (overconverging) or hyporefractive (underconverging) lens or from structural abnormalities of the eyeball

Problems of Refraction (a) In the emmetropic (normal) eye, light from both near and distant objects is focused properly on the retina

Problems of Refraction (b) In a myopic eye, light from a distant object comes to a focal point before reaching the retina and then diverges again

Homeostatic Imbalances of Refraction Myopia, or nearsightedness, occurs when objects focus in front of the retina and results in seeing close objects without a problem but distance objects are blurred Eyeball too long Correction has traditionally involved use of concave lenses that diverge the light before it enters the eye, but procedures to flatten the cornea slightly—a painless 10-minute surgery called radial keratotomy, or PRK and LASIK procedures using a laser— have offered other treatment options

Problems of Refraction (c) In the hyperopic eye, light from a near object comes to a focal point behind (past) the retina. Refractory effect of the cornea is ignored

Homeostatic Imbalances of Refraction Hyperopia or farsightedness occurs when objects are focused behind the retina and results in seeing distance objects clearly but close objects are blurred Eyeball is too short or a lens with poor refractory power (lazy lens) Convex corrective lenses are needed to converge the light more strongly for close vision

Homeostatic Imbalances of Refraction Astigmatism: –Unequal curvatures in different parts of the lens (or cornea) lead to blurry images –Special cylindrically ground lenses and laser procedures are used to correct this problem

PROBLEMS OF REFRACTION

THE EYE AND VISION Physiology of Vision: –Photoreception is the process by which the eye detects light energy Photoreceptors are modified neurons that structurally resemble tall epithelial cells Rods are highly sensitive and are best suited to night vision Cones are less sensitive to light and are best adapted to bright light and colored vision Photoreceptors contain a light-absorbing molecule called retinal –Stimulation of the Photoreceptors The visual pigment of rods is rhodopsin and is formed and broken down within the rods The breakdown and regeneration of the visual pigments of the cones is essentially the same as for rhodopsin

Functional Anatomy of the Photoreceptors Photoreceptors are modified neurons Structurally they resemble tall epithelial cells turned upside down with their tips immersed in the pigmented layer of the retina Named according to the shape of the outer segment: rod/cone shape

Functional Anatomy of the Photoreceptors Outer segments contain an elaborate array of visual pigments (photopigments) that change shape as they absorb light: –Pigments embedded in areas of the plasma membrane that forms discs In rods, discs are discontinuous— stacked like a row of pennies in a coin wrapper In cones, disc membranes are continuous with the plasma membrane; thus, the interiors of the cone discs are continuous with the extracellular space –Tips of the outer segments of rods and cones are removed (phagocytized by cells of the pigmented layer) and renewed daily

Functional Anatomy of the Photoreceptors Rods: –Very sensitive (respond to very dim light) –Best suited for night vision and peripheral vision –Absorb all wavelengths of visible light, but their inputs are perceived only in gray tones Cones: –Need bright light for activation (have low sensitivity) –Have pigments that furnish a vividly colored view of the world

PHOTORECEPTORS

Rods and Cones Wired differently to other retinal neurons Rods participate in converging pathways –Effects are summated and considered collectively –Vision fuzzy and indistinct Cones have a straight- through pathway via their own personal bipolar cell to a ganglion –Has its own line to the higher visual centers –Accounts for the detailed, high-resolution

Rods and Cones Because rods are absent from the foveae and cones do not respond to low-intensity light, we see dimly lit objects best when we do not look at them directly, and recognize them best when they move –Moonlit evening

Chemistry of Visual Pigments Light-absorbing molecule called retinal combines with proteins called opsins to form four types of visual pigments: –Depending on the type of opsin to which it is bound, retinal preferentially absorbs different wavelengths of the visible spectrum –Retinal is chemically related to vitamin A and is made from it –Liver stores vitamin A and releases it as needed by the photoreceptors to make visual pigments –Cells of the pigmented layer of the retina absorb vitamin A from the blood and serve as the local vitamin A depot for the rods and cones

RETINAL ISOMERS IN PHOTORECEPTION

Chemistry of Visual Pigments Retinal can assume a variety of distinct three-dimensional forms (isomers) Bound to opsin, retinal has a bent shape called the 11-cis isomer –When the pigment is struck by light and absorbs photons, retinal twists and snaps into a new configuration (all-trans isomer/pigment is bleached) which causes retinal to detach from opsin –This is the only light-dependent stage, and this simple photochemical event initiates a whole chain of chemical and electrical reactions in rods and cones causing electrical impulses to be transmitted along the optic nerve

Stimulation of the Photoreceptors Excitation of rods: –The visual pigment of rods is a deep purple pigment called rhodopsin –Rhodopsin molecules are arranged in a single layer in the membranes of each of the thousands of discs in the rods’ outer segments –Although rhodopsin absorbs light throughout the entire visible spectrum, it maximally absorbs green light

Stimulation of the Photoreceptors (b): shows a small segment of the membrane of a visual pigment- containing disc of the outer segment of a rod cell The visual pigments consist of a light-absorbing molecule called retinal bound to an opsin protein Each type of photoreceptor has a characteristic kind of opsin protein, which affects the absorption spectrum of the retinal In rods, the pigment-opsin complex is called rhodopsin Notice that the light-absorbing retinal occupies the core of the rhodopsin molecule

Stimulation of the Photoreceptors Excitation of Rods: –Rhodopsin forms and accumulates in the dark –Vitamin A is oxidized to the 11-cis retinal form and then combined with opsin to form rhodopsin –Rhodopsin absorbs light and changes to all-trans retinal isomer –Retinal-opsin combination breaks down Retinal and opsin separate (bleaching) –All-trans retinal reconverted with enzymes (aid of light) to 11-cis retinal isomer –Rhodospin is regenerated when 11-cis retinal is rejoined to opsin

RHODOPSIN

Stimulation of the Photoreceptors Excitation of Cones –Essentially the same as for rhodopsin but the threshold for cone activation is much higher than that for rods because cones respond only to high-intensity (bright) light –Visual pigments of the three types of cones (like rods) are a combination of retinal and opsins However, the cone opsins differ from rods and from one another The naming of cones reflects the colors (wavelengths) they absorb (e.g.: blue cones, red cones, etc) –Absorption spectra overlaps with perception of intermediates hues (orange, yellow, purple, etc) »Yellow light stimulates both red and green cones »If red is stimulated more than green, we see orange »When all cones are stimulated equally, we see white

HOMEOSTATIC IMBALANCE Color blindness is due to a congenital lack of one or more of the cone types –Sex linked condition –Most common red-green color blindness

PHOTOTRANSDUCTION

Light Transduction in Photoreceptors Light and dark Adaptation –Rhodopsin is amazingly sensitive to light (even starlight causes some of the molecules to be bleached) –As long as the light is low intensity, little rhodopsin is bleached and the retina responds to light stimuli –In high intensity light there is bleaching of the pigment and rhodopsin breaks down as rapidly as it is made Rods become nonfunctional and cones begin to respond

THE EYE AND VISION Physiology of Vision –Light adaptation occurs when we move from darkness into bright light Retinal sensitivity decreases dramatically and the retinal neurons switch from the rod to the cone system –Dark adaptation occurs when we go from a well-lit area into a dark one The cones stop functioning and the rhodopsin starts to accumulate in the rods increasing retinal sensitivity

HOMEOSTATIC IMBALANCE Night blindness (nyctalopia) –Condition in which rod function is seriously hampered –Most common cause is Vitamin A deficiency which leads to rod degeneration

THE EYE AND VISION –Visual processing occurs when the action of light on photoreceptors hyperpolarizes them, which causes the bipolar neurons from both the rods and cones to ultimately send signals to their ganglion cells

VISUAL FIELDS Physiology of Vision –Visual Pathway to the Brain The retinal ganglion cells merge in the back of the eyeball to become the optic nerve

VISUAL FIELDS At the X-shaped optic chiasma, fibers from the medial aspect of each eye cross over to the opposite side and continue on via the optic tracts: –Thus, each optic tract: Contains fibers from the lateral (temporal) aspect of the eye on the same side and fibers from the medial (nasal) aspect of the opposite eye Carries all the information from the same half of the visual field

VISUAL FIELDS Also notice that, the lens system of each eye reverses all images: –The medial half of each retina receives light rays from the temporal (lateral-most) part of the visual field (that is, from the far left or far right rather than from straight ahead) –The lateral half of each retina receives an image of the nasal (central) part of the visual field –Consequently, the left optic tract carries (and sends on) a complete representation of the right half of the visual field, and the opposite is true for the right optic tract

VISUAL FIELDS The paired optic tracts send their axons to neurons within the lateral geniculate body of the thalamus, which maintains the fiber separation established at the chiasma, but balances and combines the retinal input for delivery to the visual cortex Axons from the thalamus project through the internal capsule to form the optic radiation of fibers in the cerebral white matter –These fibers project to the primary visual cortex in the occipital lobes, where conscious perception of visual images (seeing) occurs

VISUAL FIELDS Notice that although both eyes are set anteriorly and look in approximately the same direction, their visual fields, each about 170 degrees, overlap to a considerable extent, and each eye sees a slightly different view Crossing over is not complete

VISUAL FIELDS Cortical “fusion” of the slightly different images delivered by the two eyes provides us with depth perception, an accurate means of locating objects in space (three- dimensional vision) Many animals (pigeons, rabbits, etc.) have panoramic vision. There eyes are placed more laterally on the head, so that the visual fields overlap verylittle, and crossover of the optic nerve fibers is almost complete –Consequently, each visual cortex receives input principally from a single eye and a totally different visual field

HOMEOSTATIC IMBALANCE Loss of left eye: –Nothing would be seen in the visual field area colored yellow –Loss of true depth perception –Peripheral vision is lost on the side of damage

HOMEOSTATIC IMBALANCE If neural destruction occurs beyond the optic chiasma—in an optic tract, the thalamus, or visual cortex — then part or all of the opposite half of the visual field is lost

HOMEOSTATIC IMBALANCE A stroke affecting the left visual cortex leads to blindness in the right half of the visual field –But, since the right (undamaged) visual cortex still receives inputs from both eyes, depth perception in the remaining half of the visual field is retained

OPTIC NERVE

VISUAL FIELDS

RESPONSES OF RETINAL GANGLION

THE EAR: HEARING AND BALANCE

Outer (External) Ear The outer (external) ear consists of the auricle (pinna): –Composed of elastic cartilage covered with thin skin and an occasional hair –Function is to direct sound waves into the external auditory canal

Outer (External) Ear The external auditory canal (meatus) extends from the auricle to the eardrum Passes through the temporal bone Lined with skin bearing hairs, sebaceous glands, and modified sweat glands called ceruminous glands: –Secret yellow-brown waxy cerumen (earwax) which provides a sticky trap for foreign bodies and repels insects

Outer (External) Ear Tympanic Membrane (eardrum): –Boundary between the outer and middle ears –Thin, translucent, connective tissue membrane, covered by skin on its external face and by a mucosa internally –Shapped like a flattened cone, with its apex protruding medially into the middle ear –Sound waves make the membrane vibrate: The eardrum, in turn, transfers the sound energy to the tiny bones of the middle ear and sets them into vibration

Middle Ear The middle ear, or tympanic cavity, is a small, air-filled, mucosa-lined cavity in the petrous portion of the temporal bone Flanked laterally by the eardrum and medially by a bony wall with two openings: –The superior oval (vestibular) window –The inferior round (cochlear) window Mastoid antrum (canal in the posterior wall of the middle ear) located in the roof (epitympanic recess) of the tympanic cavity allows it to communicate with mastoid air cells housed in the mastoid process

Middle Ear Pharyngotympanic (auditory) tube: –Eustachian tube –Links middle ear with the nasopharynx –The mucosa of the middle ear is continuous with the lining of the pharynx –Normally flattened and closed: Swallowing or yawning opens it briefly to equalize pressure in the middle ear cavity with external air pressure Important because the eardrum vibrates freely only if the pressure on both of its surfaces is the same –Otherwise, sounds are distorted Ear-popping sensation of the pressure equalizing is familiar to anyone

Middle Ear It is spanned by the auditory ossicles (three smallest bones in the body) Named for their shape: –Malleus: hammer –Incus: anvil –Stapes: stirrup Handle of the malleus is secured to the eardrum, and the base of the stapes fits into the oval window Tiny ligaments suspend the ossicles, and mini-synovial joints link them together Incus articulates with the malleus laterally and the stapes medially Ossicles transmit the vibratory motion of the eardrum to the oval window, which in turn sets the fluids of the inner ear into motion, eventually exciting the hearing receptors

Middle Ear Tiny skeletal muscles attached to the ossicles prevent damage due to loud sounds by restricting their movements thus limiting the movement of the stapes in the oval window

HOMEOSTATIC IMBALANCE Otitis media –Middle ear inflammation –Eardrum bulges and becomes inflamed and red –Most cases treated with antibiotics –If large amounts of pus or fluids accumulate a myringotomy (lancing of the eardrum) may be required to equalize the pressure Tiny tube implanted in the eardrum permits pus to drain into the external ear Tube falls out into the external ear in time –Non-infectious causes: food allergies (typically milk or wheat)

Inner (Internal) Ear Lies deep in the temporal bone behind the eye socket The inner (internal) ear has two major divisions: the bony labyrinth and the membranous labyrinth

Inner (Internal) Ear Bony (osseous) labyrinth: –System of twisting channels through the bone –Three regions: Vestibule Cochlea Semicircular canals

Inner (Internal) Ear Bony labyrinth is filled with perilymph –Fluid similar to cerebrospinal fluid and continuous with it

Inner (Internal) Ear Membranous labyrinth is a continuous series of membranous sacs and ducts contained within the bony labyrinth following its contour Surrounded by and floats in the perilymph Interior contains endolymph which is chemically similar to K + rich intracellular fluid These two fluids (perilymph and endolymph) conduct the sound vibrations involved in hearing and respond to the mechanical forces occurring during changes in body position and acceleration

Inner (Internal) Ear Bony labyrinth: –The vestibule is the central cavity of the bony labyrinth with two membranous sacs suspended in the perilymph, the saccule and the utricle –The semicircular canals project from the posterior aspect of the vestibule, each containing an equilibrium receptor region called a crista ampullaris –The spiral, snail-shaped cochlea extends from the anterior part of the vestibule and contains the cochlear duct, which houses the spiral organ of Corti, the receptors for hearing

Inner (Internal) Ear Vestibule: –The vestibule is the central egg-shaped cavity of the bony labyrinth –In its lateral wall is the oval window

Inner (Internal) Ear Vestibule: –Suspended in its perilymph and united by a small duct are two membranous labyrinth sacs: Saccule: –Small sac –Continuous with the membranous labyrinth extending anteriorly into the cochlea (cochlear duct) Utricle: continuous with the semicircular ducts extending into the semicircular canals posteriorly –The saccule and utricle house equilibrium receptor regions called maculae that respond to the pull of gravity and report on changes of head position

Inner (Internal) Ear Semicircular Canals: –The semicircular canals lie posterior and lateral to the vestibule and each of these canals defines about 2/3 of a circle –Cavities of the bony semicircular canals project from the posterior aspect of the vestibule, each oriented in one of the three planes of space –There is an anterior, posterior, and lateral semicircular canal in each inner ear

Inner (Internal) Ear Semicircular Canals: –The anterior and posterior canals are oriented at right angles to each other in the vertical plane, whereas the lateral canal lies horizontally –Snaking through each semicircular canal is a corresponding membranous semicircular duct, which communicates with the utricle anteriorly Each of these ducts has an enlarged swelling at one end called an ampulla, which houses an equilibrium receptor region called a crista ampullaris –These receptors respond to angular (rotational) movements of the head

Inner (Internal) Ear Cochlea: –Latin for snail –Spiral, conical, bony chamber about the size of a pea

Inner (Internal) Ear Cochlea: –Running through its center like a wedge- shaped worm is the membranous cochlear duct, which ends blindly at the cochlear apex –Cochlear duct houses the spiral organ of Corti, the receptor for hearing

Inner (Internal) Ear Cochlea: –Cochlear duct houses the spiral organ of Corti, the receptor for hearing –Divided into 3 chambers (scalas Scala vestibuli: –Contained perilymph Scala media (cochlear duct): –Contained endolymph Scala tympani: –Contained perilymph

Inner (Internal) Ear Cochlea: –Floor of the cochlear duct (Scala media) is composed of the bony spiral lamina and the flexible fibrous basilar membrane, which supports the organ of Corti –Cochlear nerve, division of the vestibulocochlear (Cranial Nerve VIII) nerve, runs from the organ of Corti through the modiolus on its way to the brain

Inner (Internal) Ear (c): detailed structure of the spiral organ of Corti

COCHLEA (d): Electron micrograph of cochlear hair cells

THE EAR: HEARING AND BALANCE Physiology of Hearing –Properties of Sound: Sound is a pressure disturbance produced by a vibrating object and propagated by the molecules of the medium Frequency is the number of waves that pass a given point in a given time Amplitude, or height, of the wave reveals a sound’s intensity (loudness) –Airborne sound entering the external auditory canal strikes the tympanic membrane and sets it vibrating –The resonance of the basilar membrane processes sound signals mechanically before they ever reach the receptors

HUMAN HEARING Sounds set up vibrations in air that beat against the eardrum that pushes a chain of tiny bones that press fluid in the inner ear against membranes that set up shearing forces on tiny hair cells that stimulate nearby neurons that give rise to impulses that travel to the brain, which interprets them—and you hear

SOUND Light can be transmitted through a vacuum (outer space) Sound depends on an elastic medium for transmission Sound travels much more slowly than light: –Sound travels in dry air: 0.2 miles/second (331 m/s) –Light travels 186,000 miles/second (300,000 km/s A lightning flash is almost instantly visible, but the sound it creates (thunder) reaches our ears much more slowly –For each second between the lightning bolt and the roll of thunder, the storm is 1/5 mile farther away –Speed of sound is constant in a given medium: It is greatest in solids and lowest in gases, including air

SOUND Sound is a pressure disturbance— alternating areas of high and low pressure—produced by a vibrating object and propagated by the molecules of the medium Tuning fork struck on left Prong will first move to the right, creating an area of high pressure by compressing the air molecules there Then, as the prongs rebound to the left, the air on the left will be compressed, and the region on the right will be rarefied, or low-pressure, area since most of its air molecules have been pushed farther to the right) As the fork vibrates alternating from right to left, it produces a series of compressions and rarefactions, collectively called a sound wave, which moves outward in all directions (b)

SOUND The individual air molecules just vibrate back and forth for short distances as they bump other molecules and rebound The outward-moving molecules give up kinetic energy to the molecules they bump, energy is always transferred in the direction the sound wave is traveling –Thus, with time and distance, the energuy of the wavw declines, and the sound dies a natural death We can illustrate a sound wave as an S-shaped curve, or sine wave, in which the compressed areas are crests and the rarefied areas are troughs ©

FREQUENCY Number of waves that pass a given point in a given time (waves/time) Sine wave of a pure tone is periodic –Crests and troughs repeat at definite distances –Distance between two consecutive crests (or troughs) is called the wavelength Shorter the wavelength, the higher the frequency

FREQUENCY Hertz: unit of frequency –Cycle/sec –Waves/sec –Hz Frequency range of human hearing is: –20 to 20,000 hertz (waves/sec) We perceive different sound frequencies as differences in pitch: –The higher the frequency, the higher the pitch –The lower the frequency, the lower the pitch Most sounds are mixtures of several frequencies: –This characteristic of sound, called quality enables us to distinguish between the same musical note from different sources (high C from singer, piano, clarinet, etc) –Provides the richness and complexity of sounds (music) we hear

AMPLITUDE Height of the sine wave reveals a sound’s intensity, which is related to it energy, or the pressure differences between its compressed and rarefied areas Loudness refers to our subjective interpretation of sound intensity –Pin drop to a steam whistle Measures in logarithmic units called decibels (dB) –Each 10-dB increase represents a tenfold increase in sound intensity –A sound of 20 dB and 0 dB has 100 times more energy but a 2-fold increase in loudness Most people would say that a 20 dB sound seems about twice as loud as a 10 dB sound Threshold of pain is 130 dB Severe hearing loss occurs with frequent or prolonged exposure to sounds with intensities greater than 90 dB –Normal conversation: 50 dB –Noisy restaurant: 70 dB –Amplified music: 120 dB or more

FREQUENCY/AMPLITUDE

Transmission of Sound to the Inner Ear Hearing occurs when the auditory area of the temporal lobe cortex is stimulated but sound must be propagated through air, membranes, bones, and fluids to reach and stimulate receptor cells in the organ of Corti Airborne sound entering the external auditory canal strikes the tympanic membrane and sets it vibrating at the same frequency –Greater the frequency, the farther the membrane is displaced in its vibratory motion Motion of the tympanic membrane is amplified and transferred to the oval window by the ossicle lever system –Acts much like a hydraulic press or piston to transfer the same total force hitting the eardrum to the oval window

Transmission of Sound to the Inner Ear Since the tympanic membrane is 17 to 20 times larger than the oval window, the pressure (force per unit area) actually exerted on the oval window is about 20 times that on the tympanic membrane –Compared heel of a man’s shoe and spiked heel of a women’s shoe This increased pressure overcomes the impedance of cochlear fluid and sets it into wave motion

SOUND WAVES

Resonance of the Basilar Membrane As the stapes rocks back and forth against the oval window, it sets the perilymph in the scala vestibuli into a similar back-and-forth motion, and a pressure wave travels through the perilymph from the basal end toward the helicotrema (opening at the tip of the cochlear canal where the scala tympani and scala vestibuli unite), much as a piece of rope held horizontally can be set into wave motion by movement initiated at one end

Resonance of the Basilar Membrane Sounds of very low frequency (below 20 Hz) create pressure waves that take the complete route through the cochlea—up the scala vestibuli, around the helicotrema, and back toward the round oval window through the scala tympani –Such sounds do not activate the organ of Corti (below the threshold of hearing)

Resonance of the Basilar Membrane Sounds of higher frequency (shorter wavelengths) create pressure waves that take a “shortcut” and are transmitted through the cochlear duct into the perilymph of the scala tympani

Resonance of the Basilar Membrane Fluids are incompressible Water bed: sit on one side, the other side bulges Each time the fluid adjacent to the oval window is forced medially by the stapes, the membrane of the round window bulges laterally into the middle ear cavity and acts as a pressure valve

Resonance of the Basilar Membrane As a pressure wave descends through the flexible cochlear duct, it sets the entire basilar membrane into vibrations

Resonance of the Basilar Membrane (b): Maximal displacement of the membrane occurs where the fibers of the basilar membranes are tuned to a particular sound frequency –This characteristic of many natural substances is called resonance Fibers near the oval window (cochlear base) are short and stiff, and they resonate in response to high-frequency pressure waves Longer, more floppy basilar membrane fibers near the cochlear apex resonate in time with lower- frequency pressure waves Thus, sound signals are mechanically processed by the resonance of the basilar membrane, before ever reaching the receptors

Excitation of Hair Cells in the Organ of Corti The organ of Corti, which rests atop the basilar membrane, is composed of supporting cells and hearing receptor cells called cochlear hair cells Hair cells are arranged functionally: –One row of inner hair cells and three rows of outer hair cells sandwiched between the tectorial and basilar membranes –Afferent fibers of the cochlear nerve (a division of the vestibulocochlear nerve VIII) are coiled about the bases of the hair cells

Excitation of Hair Cells in the Organ of Corti Hairs (stereocilia) of the hair cells are stiffened by actin filaments and linked together by fine fibers called tip-links (arrow) They protrude into the K + rich endolymph, and the longest of them enmeshed in the overlying, gel-like tectorial membrane

Excitation of Hair Cells in the Organ of Corti Physiology of Hearing –Transduction of sound stimuli occurs after the trapped stereocilia of the hair cells are deflected by localized movements of the basilar membrane Bending the cilia towards the tallest cilium puts tension on the tip-links which in turn opens cation (+ ion) channels in the adjacent shorter stereocilia –Results in an inward K + and Ca 2+ current and a graded depolarization (receptor potential) –Increases the release of neurotransmitters –Impulses to the brain for auditory interpretation –Bending away from the tallest cilium relaxes the tip-links, closes the mechanically gated ion channels, and allows reploarization

Auditory Pathway to the Brain Impulses generated in the cochlea pass through the spiral ganglia, along the afferent fibers of the cochlear nerve to the cochlear nuclei of the medulla, to the superior olivary nucleus, to the inferior colliculus, and finally to the auditory cortex The auditory pathway is unusual in that not all of the fibers from each ear decussate (cross over) –Therefore, each auditory cortex receives impulses from both ears

Auditory Processing Auditory processing involves perception of pitch, detection of loudness, and localization of sound Auditory cortex can distinguish the separate parts of auditory signals –Whenever the difference between sound wavelengths is sufficient for discrimination, you hear two separate and distinct tones

Perception of Pitch Hair cells in different parts of the organ of Corti are activated by sound waves of different frequencies and impulses from specific hair cells are interpreted as specific pitches When the sound is composed of tones of many frequencies, several populations of cochlear hair cells and cortical cells are activated simultaneously, resulting in the perception of multiple tones

Detection of Loudness Perception of loudness suggests that certain cochlear cells have higher thresholds than others for responding to a tone of the same frequency –Example: some of the receptors for a tone with a frequency of 540 Hz might be stimulated by a sound wave of very low intensity –As the intensity of the sound increases, the basilar membrane vibrates more vigorously More hair cells would begin to respond and more impulses would reach the auditory cortex and be recognized as a louder sound of the same pitch

Homeostatic Imbalances of Hearing Deafness is any hearing loss, no matter how slight Conduction deafness occurs when something hampers sound conduction to the fluids of the inner ear –Impacted wax –Perforated eardrum –Middle ear inflammation –Otosclerosis (hardening) of the ossicles Sensorineural deafness results from damage to neural structures –Age (gradual) –Noise related damage Tearing of cilia or membranes –Tumors –Cerebral infarcts (region of dead tissue due to lack of blood supply)

Homeostatic Imbalances of Hearing Tinnitus is a ringing or clicking sound in the ears in the absence of auditory stimuli –Symptom of pathological disease Cochlear degeneration Inflammation of inner and middle ear Side effect of some medications (e.g. aspirin) Phantom sound: destruction of some neurons and growth of nearby neurons whose signals are interpreted as sound Meniere’s syndrome is a labyrinth disorder (affects both the semicircular canals and the cochlear) that causes a person to suffer repeated attacks of vertigo, nausea, and vomiting –Balance is so disturbed that standing erect is nearly impossible –Howling tinnitus is common –Might result from distortion of membranous labyrinth Mixing of the perilymph and endolymph

BALANCE Mechanisms of Equilibrium and Orientation –The equilibrium sense responds to various head movements and depends on input from the inner ear, vision, and information from stretch receptors of muscles and tendons –Under normal conditions the equilibrium receptors in the semicircular canals and vestibule, collectively called the vestibular apparatus, send signals to the brain that initiate reflexes needed to make the simplest changes in position –The equilibrium receptors of the inner ear can be divided into two functional sections Receptors in the: –Vestibule: monitors static equilibrium –Semicircular canals: monitor dynamic equilibrium

Static Equilibrium The sensory receptors for static equilibrium are the maculae –One in each saccule wall and one in each utricle –Monitor position of head in space –Key role in maintaining normal head posture with respect to gravity –Respond to linear acceleration forces, that is straight-line changes in speed and direction, but not to rotation

Maculae Flat epithelial patch containing supporting cells and scattered receptor cells called hair cells –Hair cells have numerous stereocilia and a single kinocilium protruding from their apices Embedded in the overlying otolithic membrane: jellylike mass studded with tiny stones (calcium carbonate crystals) called otoliths In the utricle, the macula are horizontal, and the hairs are vertical –Responds best to acceleration in the horizontal plane and tilting head side to side –Vertical moving (up-down) movements do not displace their horizontal otolithic membrane In the saccule, the macula is nearly vertical, and the hairs protrude horizontally into the otolithic membrane –Responds best to vertical movements Vestibular nerve (division of the vestibulocochlear nerve VIII)

Activating Maculae Receptors Causing the otolithic membrane to slide backward or forward like a greased plate over the hair cells bends the hairs (e.g. your head movement in a car stopping and starting) –Bent toward the kinocilium: depolarize with increase neurotransmitter release –Bent in opposite direction: hyperpolarize with declining neurotransmitter release –In either case, the brain is informed of the changing position of the head in space Fibers of the receptor cells release neurotransmitter because of movement of the hairs

Activating Maculae Receptors When movement of the otolithic membrane (direction indicated by the arrow) bends the hair cells in the direction of the kinocilium, the vestibular nerve fibers depolarize and generate action potential more rapidly When the hairs are bent in the direction away from the kinocilium, the hair cells become hyperpolarized, and the nerve fibers send impulses at a reduced rate i.e. below the resting rate of discharge)

Dynamic Equilibrium The receptor for dynamic equilibrium is the crista ampullaris (crista), is a minute elevation in the ampulla of the semicircular canals and activated by head movement Like the maculae, the cristae are excited by head movement (acceleration and deceleration), but in this case the major stimuli are rotatory (angular) movements Gyroscope-like receptors Since the semicircular canals are located in all three planes of space, all rotatory movements of the head disturb one or another pair of cristae (one in each ear)

Crista Ampullaris Composed of supporting cells and hair cells Hair cells, like those of the maculae, have stereocilia plus one kinocilium that project into a gel-like mass (cupula) which resembles a pointed cap Dendrites of vestibular nerve fibers encircle the base of the hair cells

Activating Crista Ampullaris Receptors Cristae respond to changes in the velocity of rotatory movements of the head Because of inertia, the endolymph in the semicircular ducts moves in the direction opposite the body’s rotation, deforming the crista in the duct As hairs are bent, the hair cells depolarize and impulses reach the brain faster Bending the cilia in opposite direction causes hyperpolarization and reduces impulse generation

Activating Crista Ampullaris Receptors (d): view of the horizontal ducts from above shows how paired semicircular canals work together to provide bilateral information on rotatory head movement

BALANCE Key point to remember when considering both types of equilibrium receptors is that the rigid bony labyrinth moves with the body, while the fluids (and gels) within the membranous labyrinth are free to move at various rates, depending on the forces (gravity, acceleration, and so on) acting on them

Equilibrium Pathway to the Brain Information from the balance receptors goes directly to reflex centers in the brain stem (preventing us from falling), rather that to the cerebral cortex Impulses travel initially to one of two destinations: –The vestibular nuclear complex in the brain stem –cerebellum

Equilibrium Pathway to the Brain Vestibular nuclei: –Major integrative center for balance (brain stem: medulla oblongata) –Also, receives inputs from the visual and somatic (proprioceptors) receptors in the neck muscles –Integrates this information and then sends commands to brain stem motor centers that control eye muscles and reflex movements of the neck, limb, and trunk muscles Allows us to remain focused on the visual field and to quickly adjust our body position to maintain or regain balance

Equilibrium Pathway to the Brain Cerebellum: –Also integrates inputs from the eyes and somatic receptors –Coordinates skeletal muscle activity

Pathways of Balance After vestibular nuclear processing, impulses are sent to one of two major regions: –Areas controlling eye movements (oculomotor control) –Areas controlling the skeletal muscles of the neck (spinal motor control)

HOMEOSTATIC IMBALANCE Nausea, dizziness, and loss of balance are common and there may be nystagmus (involuntary movement of eyes) in the absence of rotational stimuli Motion sickness: –Due to sensory input mismatch Body is fixed with reference to a stationary environment (cabin on ship) But as the ship is tossed by rough seas, your vestibular apparatus detects movement and sends impulses that disagree with the visual information This confusion somehow leads to motion sickness

DEVELOPMENTAL ASPECTS OF THE SPECIAL SENSES Embryonic and Fetal Development of the Senses –Smell and taste are fully functional at birth –The eye begins to develop by the fourth week of embryonic development; vision is the only special sense not fully functional at birth German measles during this critical time: blindness or cataracts –Development of the ear begins in the fourth week of fetal development; at birth the newborn is able to hear but most responses to sound are reflexive German measles during this critical time: deafness Effects of Aging on the Senses –Around age 40 the sense of smell and taste diminishes due to a gradual loss of receptors –Also around age 40 presbyopia begins to set in and with age the lens loses its clarity and discolors –By age 60 a noticeable deterioration of the organ of Corti has occurred; the ability to hear high-pitches sounds is the first loss

MACULA

EFFECT OF GRAVITATIONAL PULL ON A MACULA RECEPTOR

CRISTA AMPULLARIS

PATHWAYS OF BALANCE AND ORIENTATION SYSTEM

THE SPECIAL SENSES. THE CHEMICAL SENSES: TASTE AND SMELL The receptors for taste (gustation) and smell (olfaction) are chemoreceptors that respond to.

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THE SPECIAL SENSES. THE CHEMICAL SENSES: TASTE AND SMELL The receptors for taste (gustation) and smell (olfaction) are chemoreceptors that respond to.

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Presentation on theme: "THE SPECIAL SENSES. THE CHEMICAL SENSES: TASTE AND SMELL The receptors for taste (gustation) and smell (olfaction) are chemoreceptors that respond to."— Presentation transcript:

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